docs(jest): add note about .babelrc (#2911)

Closes #2895

examples/with-jest/README.md

@@ -22,3 +22,5 @@ npm test
 This example features:
 
 * An app with jest tests
+
+> A very important part of this example is the `.babelrc` file which configures the `test` environment to use `babel-preset-env` and configures it to transpile modules to `commonjs`). [Learn more](https://github.com/zeit/next.js/issues/2895).